The present invention relates to thin film coating apparatuses, methods for coating a thin film, and methods for producing liquid crystal display devices employing a soft X-ray, and liquid crystal display devices produced by the methods.
Liquid crystal display devices are actively introduced in the field of such OA (office automation) instruments as personal computers and word processors since the display devices can be driven at a low voltage, are light in weight, and provide a high image quality. As the liquid crystal display devices used in these applications, devices of twisted nematic mode in which the direction of alignment of nematic liquid crystal molecules is twisted by 90xc2x0 between the surface of a pair of upper and lower electrode substrates are generally employed. Liquid crystal display devices in which twist angle of liquid crystal molecules is increased to such a large angle as 180xc2x0 to 300xc2x0 are known as super twisted nematic mode. Besides, in order to perform matrix display or color display, development of liquid crystal display devices of active matrix type twisted nematic mode using a MIM (metal-insulating layer-metal) circuit element by which ON-OFF of a large number of pixel electrodes can be performed or using a TFT (field effect type thin film transistor) circuit element became active in recent years.
In Laid-open Japanese Patent Publication No. Hei 2-2525, a technology is disclosed in which parallel X-rays are irradiated to a substrate on the surface of which a X-ray resist layer is provided, and then the substrate is immersed in a developer and a rinse to increase the pretilt angle of liquid crystal molecules to be formed on the surface of the substrate. According to the technology disclosed in the publication, an X-ray resist layer is exposed to X-rays and immersed in a developer and a rinse to form microscopic projections and depressions on the surface of the X-ray resist layer thereby to align the liquid crystal molecules.
In Laid-open Japanese Patent Publication No. Hei 8-211622, an example of apparatuses for coating a thin film employing the effect of destaticizing with a soft X-ray is disclosed.
On the other hand, in Laid-open Japanese Patent Publication Nos. Hei 8-45695 and Hei 8-124695, static eliminating apparatuses using a soft X-ray are disclosed. Apparatuses disclosed in these publications are principally to blow the air which is ionized with a soft X-ray, to objects.
In Laid-open Japanese Patent Publication No. Hei 8-50293, which was published based on an application filed a few years ago by the present inventors, methods for producing liquid crystal display devices comprising irradiating a soft X-ray to an alignment film in a gas after a rubbing treatment are disclosed. More specifically speaking, in the publication, a technology is shown in which a soft X-ray is irradiated to an alignment film activated by a rubbing treatment, to reduce the surface energy of the alignment film thereby to prevent uneven displays from occurring in liquid crystal display devices.
Since it has entered upon a period of multi-media and many letter displays and diagrammatic displays have been sought, liquid crystal display devices are inevitably proceeding toward expanded screen size, large number of pixels, and fine displays. In keeping with such circumstances, production yield of liquid crystal display devices is on the trend of being lowered to some extent. As the factors which affect the lowering of production yield, occurrence of such uneven displays that regions having a different contrast or different chromaticity appear in a part of a screen, and happening of pixel defects such as a white defect in which the pixel becomes rid of light at the time of black display and a black defect in which the pixel does not transmit the light at the time of white display can be mentioned.
In the steps for producing liquid crystal display devices, many particles such as the so-called dusts, reaction products, and broken pieces of the substrates or surrounding materials are considered to occur. The particles slip into liquid crystal display devices and become a principal cause of the uneven displays and pixel defects described above. With the advance of cleaning technology, large particles have come to be seldom slipped into the devices. However, it is still difficult to completely remove extremely fine particles such as fatty acids generated from human bodies.
Recently, in order to remove the extremely fine particles, dry washing treatments using a nozzle capable of jetting air in such a way as performed with wipers, and wet washing treatments using pure water or an organic solvent are being used. However, it is difficult to completely remove the extremely fine particles by the dry washing treatments. With the wet washing treatments, it is pointed out that an extremely small amount of impurities are included even in washing liquids, and it is also difficult to completely remove the impurities.
On the other hand, it is known that such an extremely high static electricity as 6 to 10 keV is generated in the steps for producing liquid crystal panels, particularly at the time of conveying glass substrates, or in a thin film coating step or the like. As the result, the particles come to readily adhere on the substrates by the generated static electricity, and this fact has become a leading cause of lowering the production yield of liquid crystal display devices. In connection with the static electricity, whereas a method wherein a humidifier is used to increase humidity up to 60 to 70%, or a method wherein an ion generating apparatus is installed is being used to cope with the adhering of particles described above, such methods are still insufficient. As detrimental effects by the static electricity, disconnection or short-circuit of electrodes due to electrostatic breakdown, malfunctioning of active devices, and the like can be mentioned.
As described above, in order to increase the yield in the production of liquid crystal display devices, it is important to reduce the number of particles in production facilities, to repress the adhering of particles to substrates caused by static electricity, and to remove the adhered particles by washing. However, according to conventional methods, it was difficult to fundamentally solve the problems.
In the steps for coating or forming such a thin film as a photoresist, insulating film, and alignment film, it is difficult to remove, even by washing treatments, the particles which were adhered on the surface of a substrate in the steps of coating, evaporating, and drying, since the solvent is evaporated and the film thus formed is dried in the steps after a liquid having a flowability was applied on the surface of the substrate; and the difficulty has become a particularly serious problem.
An object of the present invention is to improve such defects in the prior art as described above; to provide apparatuses and methods for coating a thin film which apparatuses and methods are efficient for eliminating the static electricity generated in the steps for producing liquid crystal display devices; to provide methods wherein the static electricity generated in the steps for producing liquid crystal display devices is efficiently eliminated to increase the production yield; and to provide liquid crystal display devices produced by such methods.
In Laid-open Japanese Patent Publication No. Hei 8-50293 mentioned above, a technology in which a soft X-ray is irradiated to the surface of a substrate after rubbing treatment is disclosed. However, as a result of diligent studies by the present inventors, it has been discovered that uneven displays can efficiently be prevented from occurring by irradiating a soft X-ray on the surface of a substrate in a step prior to a rubbing treatment in the processes for producing liquid crystal display devices, leading to the accomplishment of the present invention.
The present invention is summarized as follows:
(1) A thin film coating apparatus comprising a substrate moving portion capable of moving a substrate in the apparatus, a coating portion for forming a thin film on the substrate, and a soft X-ray irradiating portion for emitting a soft X-ray to the substrate in the substrate moving portion or substrate coating portion.
(2) The thin film coating apparatus described in (1) above wherein the energy of the soft X-ray in the soft X-ray irradiating portion is 4 to 9.5 keV.
(3) The thin film coating apparatus described in (1) above wherein the distance from the substrate to the source of the soft X-ray is shorter than 1500 mm.
(4) The thin film coating apparatus described in (1) above wherein the soft X-ray irradiating portion comprises a first and a second soft X-ray irradiating portions capable of emitting a soft X-ray to the substrate in a location at the front and back of the coating portion, respectively.
(5) The thin film coating apparatus described in (1) above wherein the thin film is a photoresist film.
(6) The thin film coating apparatus described in (1) above wherein the thin film is an insulating film.
(7) The thin film coating apparatus described in (1) above wherein the thin film is an alignment film.
(8) A method for coating a thin film comprising using the thin film coating apparatus described in (1) above, setting a substrate in a substrate moving portion, and forming a thin film on the substrate in a coating portion.
(9) The method for coating a thin film described in (8) above wherein the thin film is a photoresist film.
(10) The method for coating a thin film described in (8) above wherein the thin film is an insulating film.
(11) The method for coating a thin film described in (8) above wherein the thin film is an alignment film.
(12) In a method for producing a liquid crystal display device comprising a step for forming a transparent electrode and a circuit element of semiconductor on the surface of a pair of transparent substrates, a step for applying a photoresist on the surface of the substrates according to a predetermined pattern, a step for exposing the photoresist applied on the substrates to the light, a step for subjecting the substrates to an etching treatment, a step for releasing the photoresist from the substrates, a step for inspecting the transparent electrodes and circuit elements of semiconductor, a step for forming an insulating film on the substrates, a step for forming an alignment film on the substrates, a step for rubbing the alignment film formed on the substrate, a step for spreading spacers on the substrate, a step for applying a sealing agent on the substrate, a step for piling up the substrates, a step for providing a gap of predetermined thickness between the substrates, a step for cutting the substrates into predetermined size of substrates, a step for filling a liquid crystal material in the gap, a step for pasting a polarizing plate to the substrate, and a step for connecting a driver IC to the transparent electrodes, the improvement which comprises irradiating a soft X-ray to the substrate in a gas in at least one step prior to the step for rubbing the alignment film.
(13) The method for producing a liquid crystal display device described in (12) above wherein the irradiating energy of the soft X-ray is 4 to 9.5 keV.
(14) The method for producing a liquid crystal display device described in (12) above wherein the distance from the substrate to the source of the soft X-ray is shorter than 1500 mm.
(15) A liquid crystal display device produced by the method described in (12) above.
(16) The liquid crystal display device described in (15) above wherein the liquid crystal material is a mixture comprising at least one compound expressed by the general formula (2) or (3) 
wherein R1 and R3 independently represent a straight chain alkyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; R2 and R4 independently represent a straight chain alkyl group or alkyloxy group having 1 to 10 carbon atoms, xe2x80x94CN, fluorine atom, chlorine atom, xe2x80x94CF3, xe2x80x94CHF2, xe2x80x94OCF3, or xe2x80x94OCHF2; S1, S2, S3, and S4 independently represent hydrogen atom, fluorine atom, chlorine atom, xe2x80x94CF3, xe2x80x94CHF2, xe2x80x94OCF3, or xe2x80x94OCHF2; Z1, Z2, and Z3 independently represent xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or single bond; and A1, A2, and A3 independently represent 
Thin film coating apparatuses of the present invention are characterized by having a substrate moving portion which is capable of moving or shifting a substrate in the apparatuses, a coating portion for forming a thin film on the substrate, and a soft X-ray irradiating portion for emitting a soft X-ray to the substrate in the substrate moving portion or substrate coating portion.
Basic principle of the present invention is that the static electricity generated in the step for coating or forming a thin film can readily and efficiently be eliminated by the adsorption, on the substrate, of the gaseous ions generated by ionization of a soft X-ray having a large wave length, and adherence of particles on the coated surface and static breakdown can be prevented by the static erasing effect to increase the yield of products.
Soft X-ray irradiating apparatuses used in the present invention are not specifically restricted so far as they can stably control their output in the range of 4 to 9.5 keV in terms of the energy of a soft X-ray. Irradiation time of the soft X-ray is not specifically restricted, either. However, it is usually 0.5 second or longer and preferably 2 to 300 seconds. When the irradiating time is shorter than 0.5 second, the effects by the soft X-ray irradiation is small. While the distance through which the soft X-ray is irradiated is not specifically restricted, the distance is usually adjusted to shorter than 1500 mm and preferably from 10 to 400 mm.
Soft X-ray irradiating apparatuses used in the present invention can be used with being loaded on a spin coat type photoresist coating apparatus, insulating film coating apparatus, or alignment film coating apparatus provided with (a) a solution supplying means which is referred to as a dispenser and allows a coating solution to fall in drops on the surface of a transparent substrate having a predetermined electrode formed on one side thereof and (b) a rotatable stage for flattening, by centrifugal force, the coating solution which was allowed to fall on the surface of the transparent substrate.
In the present invention, the atmosphere in which a soft X-ray is irradiated is not restricted at all so long as it is a gas. As preferable gases, air, nitrogen gas, carbon dioxide gas, water vapor, helium, neon, argon, a mixed gas of at least one of them with oxygen, and a mixed gas thereof can be mentioned.
Soft X-ray used in the present invention is extremely weak in its capability of passing through a substance since its energy is low. The capability is in a level at which passing of the soft X-ray can readily be shielded with a transparent polyvinyl chloride plate or the like, and the soft X-ray poses no danger to human bodies and presents no problem even from the aspect of safety supervision.
Thin film coating apparatuses of the present invention using a photoresist as thin film is described below with reference to drawings.